When Hakaze is dumped on the island, she's fully dressed, and commenting about how at least she wasn't dumped in her underwear. By the time we've encountered Mahiro after he'd made contact with Hakaze, she's wearing an improvised bra and loincloth made from her clothing. Why the dramatic clothing change - aside from the possible explanation of the passage of time? Remember, to do magic powered by the Tree of Genesis, you need to sacrifice some of the products of modern society. On this island, what would Hakaze have to sacrifice? Underwear and artificial fibers.
Yoshino's Oblivious to Love state about Hakaze makes more sense when you think about it.His girlfriend who he truly loved died suddenly with no real explanation. He's never truly moved on from Aika's death as shown by the fact that he didn't cry when it first happened. Basically, Yoshino ignores Hakaze because he hasn't moved on from her death, just as Aika feared. He feels if he dated Hakaze, he'd be cheating on her in his heart.
Hanemura being the only one to figure out that Yoshino is Aika's boyfriend when everyone else around him is making wilder and wilder guesses makes sense because Hanemura is a complete outsider to all the other characters and does not have any preconceived notions of what they would be like or what they think is most likely.
Actually I figured there was another reason. Hanemura was the only one in the group who had been in a deep and meaningful relationship before. Mahiro, Samon, and Junichirou all seem to be stick in the mud, making it harder for them to look at Yoshino and see what Hanemura sees. Basically, Hanemura is Not So Different from Yoshino.